The reproduction of images by creating one or more half-tone images corresponding to one or more colors is a well-known technique; it is embodied in a variety of reproductive processes known to those of ordinary skill in the printing and graphic arts. The half-tone images are comprised of dot-patterns or other similar patterns. By overlaying the half-tone images, an accurate reproduction of the original image can be produced in large quantities. Printing processes such as photogravure, lithography and flexography all use such dot-pattern, half-tone images. Each half-tone image may be created photographically by superimposing a contact screen possessing such a dot-pattern on the original image. Other methods of obtaining half-tone images, such as those employing computer-controlled optical or laser devices which do not use screens, are also known within the art. The displays of cathode ray, liquid crystal display and other electronic devices also create a reproduction of an image by displaying a dot-pattern representation. These dots are usually known as pixels.
One method of printing, commonly known as gravure printing, is generally accomplished using plates containing fine recesses which hold ink or dye. Images may be reproduced in multiple colors by using a color separation process involving multiple half-tone images. By creating several half-tone copies of an original, continuous-tone image, the primary colors may be separated by photographic or other techniques and converted into several dot-pattern images. Thus, a continuous tone image is converted into several half-tone images with a high density of dots, typically 133 dots per inch. The resulting half-tone images may then be overlaid to reproduce the original image.
One technique for creating the half-tone color separations involves placing a dot-pattern screen on photographic film and making a contact negative of the continuous tone image. Alternatively, computer-controlled lasers may be employed to create the negative without employing a screen. Each dot-pattern negative is then used to print a half-tone image containing a different color. By overlaying dot-pattern images of different colors, a color image approximating the original continuous tone image results.
Half-tone images may also be created using computer-controlled scanners which utilize either an optical beam or a laser to impart a dot-pattern onto a photographic plate or similar means. A laser device can also be used to incise a dot-pattern directly upon a photogravure plate, which may either be flat or cylindrical. These processes do not utilize screens, but nonetheless require the original image to be converted into one more dot-pattern half-tone images in order for reproductions to be made.
The screen of a cathode ray tube contains discrete dots or line segments which are selectively activated to reproduce an image. Similarly, liquid crystal displays contain elements which are activated electronically to create a reproduction of an image which is composed of a dot-pattern, in the manner of a half-tone image. Other types of displays operate in a manner which also utilizes the technique of breaking an image into a discrete pattern of activated and inactivated dots or line segments to create a reproduction.
Those of ordinary skill in the art will understand that the term "dot-patterns" generally encompasses both positive and negative patterns. The "dot-patterns" used in a reproduction process may be comprised of dots, line segments or other discrete areas which are to be substantially filled with color along with other discrete areas substantially free from the color being printed. The term "printing", as related to the present invention, is recognized as a broad term, generally describing those reproduction processes used in the graphic arts to transfer an image to an object. The term "printing" is therefore meant to include techniques such as gravure printing, lithography, flexography, photoduplication and any and all techniques which reduce an image to discrete areas of color (i.e., dot-patterns) commonly known as a half-tone images. It is further generally understood that "printing" encompasses using one or more colors. For example, in four color reproduction, the most common are the three primary colors (magenta, cyan, and yellow) and black. Therefore, numerous combinations of printing methods, dot patterns and color combinations exist. All of these however, share the common element of reproducing a continuous tone original image by creating one or more dot-pattern half-tone color separations. In these processes, both shading and hue are governed by the arrangement and density of the discrete areas arrayed to form the dot-pattern.
In order for the any of above-described technique to be successful, the dots of different colors should be separated. Failure to provide this separation will result in distortion caused by the different color inks or dyes running together. One manner in which the necessary separation has previously been accomplished is by using screens which consist of lines or dots that are periodically spaced. By rotating these screens with respect to one another, e.g., by about thirty degrees, dots of different colors are reproduced which are sufficiently separated to reduce color distortion.
It is known to those skilled in the art, however, that overlaying periodic dot patterns will generally produce a Moire interference pattern. These patterns are caused by an optical effect (Moire effect) which occurs when one or more periodic patterns are superimposed upon each other, but rotated by a finite angle such that the patterns cross at angles to produce a periodic pattern of interference lines, whose spacing varies with the angle of rotation. The optical effect thereby induced causes a new family of curves to appear which passes through the intersections of the original patterns. As applied to the dense dot patterns of multi-color printing, the Moire effect produces an objectionable interference pattern (e.g., checkerboard, rosettes, wavy lines, etc.) which can destroy the effect of an image by distorting the perception of the texture, color and detail in the finished work. Moire patterns even occur in flat screen tints which use one or more colors of ink to produce other color values. Interference patterns also appear on images created using cathode ray tubes and other types of electronic displays. The image displayed, for example a television picture or computer graphics, is composed of individual elements, or pixels, arrayed substantially in a periodic pattern. For this reason, Moire and similar interference patterns appear and degrade the quality of the reproduction.
One partial solution to the problem of Moire interference patterns in the graphic arts has been to register the different dot-pattern screens at precise, empirically determined angles with respect to one another. Even if done correctly, this technique will not eliminate the Moire patterns, but merely cause the spacing between the Moire lines to be greater than the dimensions of the image. This is obviously a severe practical limitation on the use of registration as a means of reducing the Moire effect. Additionally, this technique is exceedingly difficult to accomplish when printing on paper, metal or plastic, and is essentially impossible when printing on textiles, due to the inherent irregularities in the material.
Others have recognized and attempted to provide a solution to this problem. For example, U.S. Pat. No. 4,553,215--Masuda et al., attempts to solve the problem by displacing the sides of periodic screens in an irregular manner to produce irregularly varied shapes, or irregular sets of points corresponding to the vertices of these irregular shapes. The patterns of Masuda et al. begin with a pattern of one of the three known, regular, space-filling polygons, (the regular triangle, the regular quadrilateral, and the regular hexagon), to which some irregular deformation is imparted. Because the deformations are obtained by bounded deviations from a periodic pattern, the Moire interference pattern may only be reduced, not completely eliminated. It is well known that unbounded randomness will produce undesirable fluctuations in dot density, thereby creating a less-than-optimal printed image.
As a variation on the concept of Masuda et al., non-periodic deviations, such as those known in quasiperiodic systems theory, might be considered. However, in this case, such patterns retain a crystallographic (i.e., square, rectangular, triangular or rhombic) orientational symmetry. Again, as with Masuda et al., since the translations from periodicity are bounded, the Moire effect cannot be completely eliminated. This technique, as well as that disclosed by Masuda et al., further suffers from the disadvantage of creating an almost infinite number of fundamental elements which define the resulting dot-pattern. Preferably, two or three fundamental elements should be defined which may me arrayed to produce a dot-pattern which eliminates Moire interference.
The use of irrational numbers to generate dot-patterns has been disclosed in U.S. Pat. No. 3,997,911--Perriman et al. However, the irrational numbers are not used to produce a quasiperiodic pattern. Instead, irrational numbers are used as part of an efficient algorithm used to rotate a periodic patterns so as to produce a new periodic pattern, where the tangent of the angle of rotation between the patterns is irrational. Since the resulting pattern is periodic, superimposing this pattern over the original periodic pattern produces the Moire effect. Rather than eliminating the Moire interference pattern, the goal of Perriman et al. is to apply irrational numbers in an automated procedure to rapidly obtain from an original image a sequence of periodic patterns at optimal registry angles. In other words, this technique implements the traditional registry approach whereby although a Moire pattern is produced, the angles between the periodic patterns are adjusted such that the wavelength of the Moire interference is large relative to the image.
From these examples, it can be seen that prior attempts to eliminate Moire interference patterns have not been entirely successful. Instead, those skilled in the art have had to be content with minimizing the Moire effect by utilizing complex deviations from periodic patterns or by careful registry--with a resultant trade off in image quality as a result. Hence, there remains a long-felt and unfulfilled need for a generalized and simplified technique which may be used to generate half-tone color separations which will render accurate dot-pattern images and eliminate the undesirable Moire effect.